Spondylolisthesis consists of the slipping of one vertebra over another either forwards (anterolisthesis) or backwards (retrolisthesis) which in any case determines that a vertebra is misaligned with the rest, causing pain and nerve compression.
If the decision is taken to operate to correct this condition, normally screws are fixed to various vertebrae and connected by means of a bar which is subjected to traction to fix or to try to correct the misalignment between vertebrae.
Patent application PCT/ES00/00310 belonging to the applicant for this invention proposes a system for inter-vertebral fixation for spine treatments, the purpose of which is to allow the implanting of a system of tulips that are physically independent of the rest of the pieces involved in it, allowing the implantation to be made in optimal conditions from the point of view of manipulation as well as from the visual point of view.
The invention fundamentally comprises a screw with a spherical head, a tulip element that can receive the head of the screw, a lower boss which is fixed to the screw and that has a housing on which the bar rests, with the bar protruding through recesses in the tulip, the bar being conveniently fixed by means of another upper boss on which a threaded cap is located that allows for the definitive fixing of the assembly is located. There is a cavity in the spherical head of the screw into which an Allen key can be inserted to facilitate the implanting of the screw in the bone before the rest of the device mechanisms are implanted.
When it is necessary to cope with level differences between vertebrae, normally three systems of screws of different lengths associated with the bar are used; however, it may occur that once a screw length has been chosen, it may require another length and if the screw is changed, it must be for one of a larger diameter, which is sometimes not possible since it does not fit in, resulting in an awkward undertaking during the operation. Another system consists of using tulips that are longer than necessary and that must be cut once they have been installed, which causes another inconvenience during the operation; finally, there are those who advise the bending of the bar in situ, a very laborious undertaking when carried out during the operation, during which the operating time is very important.